Bifunctional alumina-based catalysts comprising a sulfactive metal hydrogenation component and an acidic promoter such as silica, halogen, phosphate, etc. are well known in the art. The sulfactive metal hydrogenation component comprises Mo and/or W plus Co and/or Ni, in sulfided form and is active primarily for the selective hydrogenation and cracking of organic sulfur and nitrogen compounds in hydrocarbon feedstocks, with minimum hydrocracking of hydrocarbons. With no added acidic component, such catalysts are very useful for the hydrodesulfurization of mineral oils and fractions thereof. However, it has been known for some time that the activity of such catalysts for denitrogenation of such feedstocks, or for the selective hydrocracking of high-boiling hydrocarbons, can be much improved by incorporating therein one or more of the above-noted acidic components, fluorine being the most active.
The manufacture of such fluorine-promoted catalysts has in the past entailed considerable expense, attributable to the difficulty involved in incorporating all of the active components uniformly into the alumina support without bringing about a drastic reduction in surface area of the final composite. The most economical method for the manufacture of multi-component, alumina-supported catalysts consists in comulling alumina hydrogel with soluble or insoluble precursors of the active components, then extruding, drying and calcining the composite. The alumina gel used in this method (usually predominantly boehmite) is highly reactive until a stage in the calcination step is reached at which it is converted to gamma alumina, which is must less reactive. When acidic fluorine compounds are present during the mulling, drying and initial stages of calcination, extensive reaction with the alumina gel takes place, with resultant marked reduction in surface area of the calcined composite. It is hence necessary to add the fluorine component after conversion of the alumina support to suitably porous, high-surface-area aggregates of gamma alumina, and this entails impregnation with aqueous solutions.
Salts of nickel and cobalt also tend to react with alumina hydrogel during mulling, drying and calcination to form relatively inactive crystalline aluminates, and hence it is also desirable to employ aqueous impregnation techniques for adding those metals. By observing proper control of calcination temperatures (to avoid loss in surface area) molybdenum or tungsten compounds can be comulled with the alumina hydrogel, extruded and calcined to provide a suitable base for subsequent impregnation with the fluorine, cobalt and nickel components, but in general highest activity is obtained by properly controlled impregnation of all components on the performed gamma alumina support.
When resorting to impregnation with a plurality of active components, a primary objective is to reduce the number of impregnations to a minimum--one if possible--since each impregnation and calcination involves substantial time and expense. However the preparation of stable impregnation solutions containing the necessary concentrations of fluorine compound and metal compounds to give the desired proportion of each component, evenly distributed throughout the pores of the alumina support from a single impregnation step, has not in the past been achieved. Commonly, at least two, and often three separate impregnations, with intervening calcinations have been utilized, as disclosed for example in U.S. Pat. No. 2,760,907. Insofar as I am aware a separate impregnation with the fluorine component, e.g. HF or H.sub.2 SiF.sub.6, has always been considered a practical necessity in order to insure homogeneous impregnation and prevent precipitation of one or more of the metal components in the outer rind of the support particles, the fluorides of Ni and Co being only slightly soluble in water.
I have now discovered that the fluosilicate salts of cobalt or nickel can be very advantageously employed in aqueous impregnation solutions as combined sources of active fluorine, cobalt and/or nickel in the finished catalysts. These fluosilicates, NiSiF.sub.6.6H.sub.2 O and CoSiF.sub.6.6H.sub.2 O, are very soluble in water, relatively non-corrosive, not highly acidic, and form stable solutions which do not form precipitates upon contact with the gamma alumina support. Moreover, upon calcination an additional acid-forming component is generated, SiO.sub.2, which also may improve thermal stability of the catalyst.
According to one modification of the invention a soluble Mo and/or W compound can also be included in the fluosilicate impregnation solution, e.g., ammonium metatungstate or ammonium heptamolybdate. It is preferred however that the Mo/W component be added to the support prior to impregnation with the fluosilicate solution, as by pre-impregnation of the calcined Al.sub.2 O.sub.3, or by comulling suitable Mo/W compounds with the alumina hydrogel prior to extrusion and calcination. In the latter procedure only a single impregnation is required to produce the finished catalyst, but somewhat higher activity generally results from the former procedure which requires two impregnations. According to another non-preferred technique, the Mo/W component can be impregnated into the catalyst after impregnation with the fluosilicate solution.